The construction of a roadway typically requires the preparation of a base or sub-base course which receives and supports the one or more layers of pavement materials. A base course may be composed of a variety of materials, such as soil, clay, sand, pulverized shell or the combination of such materials. The base course may also be stabilized with cements, limefly ash or other such products, or by mechanical means. The pavement materials applied to the base course to complete the roadway may include one or more layers of hot-mix asphalt, asphalt cement and aggregate, and concrete, among others.
In many instances, before the pavement may be applied to the base course, the surface of the course must be sealed and also frequently primed to receive the pavement materials. A coating of a suitable sealant is employed to ensure that the moisture content of the base course is sufficient to maintain the course in a compacted and stable condition. If this moisture content is not maintained within acceptable limits, the base course may deteriorate over time requiring repair and additional grading prior to placement of the finished pavement.
Similarly, in many applications involving asphaltic pavements, the unpaved earthen surface of the base course must be primed with a suitable material to encourage the fusion or fluxing of the pavement material with the surface of the base course. Insufficient adhesion between the base course and roadway pavement material frequently is the cause of pavement separation and cracking during installation, and subsequent failures and premature deterioration of the pavement surface. Such failures may cause dangerous traffic conditions that require costly repairs.
Cutback asphalts and emulsions of asphalts and water have been used in the past to seal and prime roadway base courses. The asphalt emulsions include products with asphalt residues in a range of from about 30% to 60% or greater by weight of the total product. The cutback asphalts typically include asphalt residues in excess of 60% by weight of the total product combined with solvents such as napthas, kerosenes, oils or other such products to maintain the asphalt in a liquid, flowable condition.
A number of difficulties, however, limit the utility of the prior art sealing and priming methods. A relatively thick coat of both the cut-back asphalts and the asphalt emulsions is normally required to provide an effective sealer coat on a base course, and such products require relatively long cure times (as long as 2 to 7 days or more) before traffic may be permitted to pass over the treated area. In many states, the prior art sealers must further be covered with sand, fines, a protective layer of sand bituminous mixture or with pavement materials during the curing process to prevent damage to the sealer and pick up of the sealer by vehicular traffic passing over the sealed surface. Both the extended cure time and the necessary application of a cover layer significantly increase the cost and the difficulty of using such products.
Moreover, the tendency of asphalt sealers to adhere to the tires and wheels of vehicles passing over the sealed surfaces frequently results in the loss of the prior art sealer coatings on entire sections of the base course. This loss of portions of the sealed surface substantially interferes with the ability of the prior art coatings to seal and prime the surface of the base course. As a result, multiple applications of such the asphalt sealers are often required to repair the sealed surface.
The use cutback asphalts and asphalt emulsions, in addition, may require equipment adapted to maintain those products at an elevated temperature throughout the application procedure. Such equipment is often expensive to operate and difficult to maintain. Furthermore, as the cutback asphalts and some asphalt emulsions cure, solvents that have been proven to damage the environment are released into the atmosphere. As a result, such products are, or will be, prohibited from general usage in many states. The prior art asphaltic products are also frequently sensitive to alkali and other soil conditions which prevent the product from properly adhering to the base course.
Petroleum products containing asphalt or asphalt derivatives have also been used as dust palliatives, i.e. a treatment to reduce airborne soil particles and soil erosion. These include resinous petroleum oil fractions that are applied to deeply penetrate unpaved roadway surfaces and other exposed soil surfaces. These dust palliatives are formulated to coat individual dust particles, to the extent possible, and to bind the soil particles together without the formation of a substantial surface coating or crust. Such crusts or coatings typically crack, separate or disintegrate with exposure to the elements to release fugitive dust and permit undesirable erosion. As a result, dust palliatives are normally applied at a rate and concentration adjusted to achieve the maximum long term penetration and to minimize the formation of crusts or coatings by the palliative.
Examples of dust palliatives of resinous petroleum oils include those discussed in U.S. Pat. No. 2,646,361, and those sold under the "Coherex" tradename. Other such products are sold by Blacklidge Emulsions, Inc., Gulfport, Miss., and Dust Allayers, Incorporated, Mansfield, Ohio, under the tradename "Dust Bond" and by Shell Oil Company under the tradename "Shellflex Oils."
Another dust palliative comprises a combination of resinous petroleum oil fractions and an asphalt in a highly diluted emulsion, i.e. one part oil and asphalt in about 6 to 15 parts water. Such products include "Dust Bond Dark" supplied by Dust Allayers, Incorporated. These products are applied at dilutions and rates to encourage their penetration into an exposed soil surface to prevent soil erosion and the creation of fugitive dust.